The invention is directed to a communication system.
Such a communication system is disclosed by European reference EP-0 433 256-A2. In this communication system, a cordless terminal equipment is normally authorized for connections via a specific cordless system unit. Cordless system units serve for connecting cordless terminal equipment to a communication network. Such a communication network can be a private branch exchange that is in turn usually connected to a regional or global network. In some instances, a cordless system unit itself can fulfill jobs of a private branch exchange.
A cordless system unit controls the recognition of the presence of an operational cordless terminal equipment in the transmission and reception area of one of its base stations and the routing of a connection setup to this terminal equipment via this base station (roaming according to ETS 300 175-3). To this end, it contains a check means for checking whether a connection setup from it to a cordless terminal equipment is possible. Such a connection setup is possible when an information exchange between at least one of the base stations of the cordless system unit and the cordless terminal equipment can be technically implemented via the cordless transmission/reception link. Given an information exchange by analog signal transmission, one criterion therefor can be the achieving of an adequately high reception signal level and, given an information exchange by digital signal transmission, can be the achieving of an adequately low bit error rate. Whether a connection setup is possible is essentially dependent on whether the cordless terminal equipment is operationally located in the transmission and reception area of a base station of the cordless system unit.
Given an existing connection between the cordless terminal equipment and a communication network connected to the cordless system unit, the cordless system unit--in conjunction with its base stations--controls the connection setup via the one base station and the new connection setup via the other base station given an area change of the cordless terminal equipment from one base station to a neighboring base station (handover according to ETS 300 175-3).
In order to monitor the access authorization of a cordless terminal equipment to the communication system and in order to monitor whether a cordless terminal equipment is authorized to initiate a connection setup via the cordless system unit, the check means checks the permissibility of such a connection setup. A cordless terminal equipment is usually authorized to initiate a connection setup via a cordless system unit when this is administratively allocated to it as home system unit. When a cordless system unit is allocated to a cordless terminal equipment as home system unit, then its identification is stored in it.
Base stations serve first and foremost for the bidirectional conversion of cordlessly transmitted signals and signals transmitted wire-bound. For example, a cordless signal transmission can be undertaken by infrared signal or by radio. For example, the DECT method described in the ETS 300 175 can be cited as a radio transmission method. A cordless system unit can have one or more base stations.
When, in a communication known from European reference EP-0 433 256-A2, a cordless terminal equipment is to be operated in the transmission and reception area of a cordless system unit that is not allocated to it as home system unit, then it must be reported at the administration of this system unit. To this end, every cordless system unit keeps a defined plurality of telephone numbers on hand that cannot be assigned to permanently allocated terminal equipment.
In such a communication system, consequently, a cordless terminal equipment can only be operated in the transmission and reception area of its home system unit or in the transmission and reception area of a cordless system unit at which it is temporarily administratively reported. The initiation of a connection setup by this cordless terminal equipment is deemed allowable by the check means of a cordless system unit only in these cases. The number of terminal equipment that can be signed on per administration for a cordless system unit in addition to the permanently allocated cordless terminal equipment is limited by the number of telephone numbers offered for these purposes. Since the individual cordless system units must offer additional free telephone numbers for the temporary logon of cordless terminal equipment, the installed resources of the communication system cannot be optimally exploited.